This application is based on Japanese Patent Application No. 11-250762 (1999) filed Sep. 3, 1999, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a printing head comprising a plurality of electrically driven printing elements and a printing apparatus using the printing head.
2. Description of the Related Art
Printing head of this kind include, for example, ink jet printing head for ejecting an ink from ink ejection opening. Ink jet printing method using such ink jet printing head have the advantages of being able to reduce noise during printing down to a negligible level, achieving fast printing, enabling printing by fixing an ink to what is called plain paper without the needs for special processing, and the like.
Of these ink jet printing methods, for example, those described in Japanese Patent Application Publication No. 54-51837 (1979) and German Patent Application Laid-open No. 2843064 (DOLS) have characteristics different from those of the other ink jet printing methods in that thermal energy is caused to act on a liquid to obtain a motive power for ejecting droplets. That is, in the printing methods disclosed in the above publications, the liquid, on which the thermal energy has acted, is subjected to changes in its conditions including a rapid increase in its volume, and the acting force based on the condition changes causes the liquid to be ejected from an orifice at a tip of an ink jet printing head, forming flying droplets. The droplets are deposited on a printing medium for printing.
In particular, the ink jet printing method disclosed in German Patent Application Laid-open No. 2843064 (DOLS) is very effectively applied to what is called a drop-on-demand printing method. Further, by using a full-line type ink jet printing head for this printing method to increase printing density, a multiorifice ink jet printing head can be easily embodied to enable fast printing of high-resolution and high-quality images.
The ink jet printing head applied to this printing method includes a print head base comprising a liquid ejection portion and a heat-generating resistor. The liquid ejection portion has an orifice provided to eject the liquid and a channel that is in communication with the orifice and that partly constitutes a heat acting portion where thermal energy used to eject droplets acts on the liquid.
Recent print head bases as described above each comprise heat-generating resistors, drivers, shift registers, and latch circuits on the same substrate. The plurality of heat-generating resistors are arranged in a line. The drives correspond to these heat-generating resistors on a one-on-one basis to drive them depending on image data. The number of shift resistors is such that they provide as many bits as the heat-generating resistors to output serially input image data parallel to the drivers. The latch circuits temporarily store the data output from the shift registers.
The configuration of a circuit in such a conventional print head base 12 is shown in FIG. 9.
In FIG. 9, reference numeral 1 denotes a plurality of heat-generating resistors arranged in a line, reference numeral 2 denotes a power transistor array functioning as a driver, reference numeral 3 denotes a latch circuit, and reference numeral 4 denotes a shift register. Reference numeral 5 denotes a terminal for accepting inputs of clock signals for shifting in data, and reference numeral 6 denotes a terminal for accepting inputs of serial printing data signals. Reference numeral 7 denotes a latch signal input terminal, and reference numeral 8 denotes a heat pulse signal input terminal for externally controlling on times for transistors in the power transistor array 2. Reference numeral 9 denotes a logic power terminal, and reference numeral 10 denotes a ground terminal. Reference numeral 11 denotes a power (VH) input terminal for driving the heat-generating resistors.
The printing head including the print head base 12 configured as described above is provided in a printing apparatus. In the printing apparatus, serial printing data are serially input to the shift register 4 from the input terminal 6. The printing data set in the shift register 4 are latched in a latch circuit 3 in response to a latch signal input from the terminal 7. When a pulse is input from the heat pulse input terminal 8, a power transistor in the transistor array 2 having the printing data set to xe2x80x9c1xe2x80x9d is turned on. Then, a heat-generating resistors 1 corresponding to the power transistor is electrically driven. The liquid (ink) in a channel in which the driven heat-generating resistor is located is heated, and the ink is ejected from an ink ejection opening corresponding to the channel for printing.
The energy required to bubble the liquid in contact with the heat-generating resistor will be considered. With constant head radiation conditions, the energy required for the bubbling is the product of energy required for the heat-generating resistor per unit area and the area of the heat-generating resistor. Thus, to obtain the energy required for the bubbling, a voltage applied to opposite ends of the heat-generating resistor, a current flowing through the heat-generating resistor, and time (a pulse width) may be set. In practical use, a constant voltage can be obtained from a power source on the side of the printing apparatus body. The current value, however, varies among bases in different lots. This is because the heat-generating resistors have different resistance values due to variations in their thickness which may occur during a process for manufacturing bases. Accordingly, if the width of the power voltage pulse to be applied to the heat-generating resistor is constant but the resistance of the heat-generating resistor increases above a set value, the current value decreases and the introduced energy becomes insufficient, thereby preventing the ink from being normally bubbled. On the contrary, if the resistance of the heat-generating resistor decreases to increase the current flowing therethrough above the set value, an excessive amount of energy is introduced to burn the heat-generating resistor or reduce its lifetime. To avoid this, a sensor may be used to monitor the resistance value of the heat-generating resistor so that the width of the pulse applied to the heat-generating resistor can be varied depending on the resistance value, to controllably keep the applied energy constant.
Next, the amount of droplets ejected from the ink ejection openings will be considered. This amount is principally related to the bubbling volume of the ink. The bubbling volume of the ink varies with the temperature of the heat-generating resistor and its periphery. Thus, before a heat pulse applied to the heat-generating resistor to eject the ink (this pulse is hereafter also referred to as a xe2x80x9cmain heat pulsexe2x80x9d) is applied, a heat pulse for applying energy insufficient to eject the ink (this pulse is hereafter also referred to as a xe2x80x9cpreheat pulsexe2x80x9d) may be applied. By adjusting the temperature of the heat-generating resistor and its periphery depending on the width of the preheat pulse or its application timings, a constant amount of droplets can be ejected to maintain a printing grade.
According to the above described prior art, the variation of the resistance value of the heat-generating resistor 1 can be corrected and the temperature of the base 12 can be controlled by feeding back signals from the sensor which are used to monitor the resistance value and the temperature. That is, heat pulse signals (drive signals for the heat-generating resistor 1) are output so that the widths of the main heat pulse and preheat pulse applied to the heat-generating resistor 1 and those pulse application timings are varied based on the feedback signals under the control of the printer apparatus body. However, other factors, for example, variations in the area of orifice openings (the ink ejection openings) or in the thickness of protective films for the heat-generating resistors 1 which may occur during manufacturing may lead to variations in the amount of ink ejected from each ink ejection opening. As a result, the density of printing images may become irregular or unwanted stripes may be formed therein. Therefore, the amount of ejected ink must be controlled for each nozzle (each ink ejection port) or each group of several nozzles.
Furthermore, due to an increase in the number of nozzles in the ink jet printing head, a plurality of the print head bases 12 may be connected together in series to constitute a multinozzle ink jet printing head. In this case, the heat-generating resistors 1 of the print head bases 12 have slightly difference resistance values. Thus, the heat pulse (including the main heat pulse and the preheat pulse) must be varied for each of the bases 12 so as to introduce about the same amount of energy to each base 12. If the printing head is constructed using the plurality of bases 12 in this manner, there will be a significant difference in print density among printed portions of the image corresponding to each base 12. Accordingly, the correction of the amount of ejected ink for each nozzle in the base 12 is more important to this printing head than to a printing head constructed using a single base 12.
The present invention is provided in view of the above described conventional examples, and it is an object thereof to provide a printing head and a printing apparatus that can correct variations in characteristics of each printing element of a printing head to print high-grade images.
It is another object of the present invention to provide a printing head and a printing apparatus wherein if printing elements can use thermal energy generated by thermoelectric converters to eject an ink from nozzles, a voltage applied to the thermoelectric converter can be variably set for each nozzle depending on characteristics of the printing element in order to correct the amount of ejected ink for each nozzle.
In the first aspect of the present invention, there is provided a printing head comprising a plurality of printing elements electrically driven through wirings based on printing data, wherein:
a plurality of wirings with different wiring resistances are connected to at least one of the printing elements so that at least one of the plurality of wirings can be selected to conduct current through the printing element.
In the second aspect of the present invention, there is provided a printing apparatus comprising:
a head installation portion in which the printing head as claimed in claim 1 can be installed, and
moving means for relatively moving the printing head and a printing medium.
According to the present invention, a plurality of wirings with different wiring resistances are connected to an electrically driven printing element, and at least one of the plurality of wirings is selected to conduct current through the printing element. Thus, drive conditions for the printing element such as an applied voltage can be variably set to correct an image printing condition for each printing element, thereby providing high-grade images free from unwanted stripes or irregular densities.